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peytonrose1015

CFM question, etc Duct size. Help.

ILoveRed
6 years ago

New construction. 36" 6 burner Wolf rangetop.

Recieved quote for my 42" Modernaire Stainless hood with exterior Abbaka blower. Two 90 degree turns in run, about 12 ft out to exterior wall. I noticed on the quote it read 1000 CFM.

Wait....originally the gal told my KD we needed 1200 CFMs. as did several here. It looks to me like this Abbaka blower has two choices...1000CFMs or 1400 CFMs.

With an external blower a distance away, wouldn't my CFMs have needed go up, not down?

she told my kd that the 10" round duct wouldn't fit in the 12" deep top of the hood so this required dropping down to an 8" duct to fit into the hoodtop, necessitating lower CFMs. This can not be right.

i had a screwed up, inadequate vent in my last house. A beautiful wood mantle hood that was totally, absolutely worthless. I just want to get this one right.

Begging for help here. Builder is all over my arse wanting to get their prepped.

Thank you.

Featured Answer
  • PRO
    Trevor Lawson (Eurostoves Inc)
    6 years ago

    Either ModernAire didn't understand the question or your KD did not understand the answer. As a distributor for Modernaire hoods in 22 US states, I can assure you a 12" depth top will accommodate a 10" round duct.

    Comments (53)

    • ILoveRed
      Original Author
      6 years ago

      I'm sorry if I'm being repetitive. I'm just feeling frustrated. I wish I was dealing with more knowledgeable folks...

    • beckysharp Reinstate SW Unconditionally
      6 years ago

      ILR, try the Kitchen forum if you haven't already.

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    • User
      6 years ago
      last modified: 6 years ago

      1200 CFM is overkill for that range anyway. Unless there’s a grill? Or the only thing you ever cook is blackened cayenne peppers? 600/900 CFM would be fine. Tuning it down a bit will be Ok.

      ILoveRed thanked User
    • ILoveRed
      Original Author
      6 years ago
      last modified: 6 years ago

      Sophie..there is no grill or griddle. And I'm just an everyday average cook.

      If you are telling me that 1000 CFMs will be fine then the Abbaka 1000 CFM is fine by me.

      Now the duct size....

      Please look at this sheet of specs. We are going with the PS26..42" width. The 4th one from the bottom. Am I reading this right? The duct has to be 9 7/8" ?

      Thank you for helping me with this.

      http://www.modernaire.com/modernaire/PS-26_files/PS2630%20Cut%20Sheet%20PDF.pdf

    • PRO
      Victory Range Hoods
      6 years ago

      The 1000 CFM with the 8" duct should be sufficient, as that is quite a bit of power to begin with. As Sophie Wheeler mentioned above, unless you are doing very heavy cooking, it should be okay to use the 8" duct with 1000 CFM. If the blower that you will be using has a capacity of 1000 CFM, the 8" duct will work well, but if the blower has the capacity to reach 1400 CFM and is trying to push 1400 CFM out of an 8" duct, it will only create extra noise. It seems that the two choices are either a 1000 CFM blower with an 8" duct or a 1400 CFM blower with a 10" duct.

      ILoveRed thanked Victory Range Hoods
    • ILoveRed
      Original Author
      6 years ago

      Victory..

      "it seems that the two choices are either a 1000 CFM blower with an 8" duct or a 1400 CFM blower with a 10" duct."

      Yes! But the information is so conflicting because the Modernaire specs seem to indicate that a 9 7/8" duct is required. See the link I posted to Sophie.


    • ILoveRed
      Original Author
      6 years ago

      You mentioned an undersized duct being noisy...this concerns me too.

    • PRO
      Victory Range Hoods
      6 years ago

      It looks like the PS2630-042 (which you will be using) indicates that the CFM levels will be based upon the connected external blower's output. The 9 7/8" dimension seems to be the distance from the back of the unit to the center of the duct, not the duct size itself. If you decide to use an 8" duct, we would suggest looking for an external blower that has no more than 1000 CFM, which should be sufficient for your needs. We hope this helps!

    • ILoveRed
      Original Author
      6 years ago

      Am I reading this wrong Victory? I'm not trying to second guess you..please forgive me, since you are trying to help me. I just want to get this right.

      The back of the unit to the center of the duct looks like "G" on the schematic which is 6" on the table.

      ....as if this is telling the contractor where to put the hole (for lack of a better word). It looks like it sits further back..rather than centered.

      am I reading this wrong?





    • weedmeister
      6 years ago

      Your picture does not have column headings.

      If 9 7/8" is H, the diameter of the opening, then that is practically 10". Once you crimp down the 10" duct, it will be around 9 7/8" in diameter.

      If you chose to go with an 8" duct, you'll need an adapter.

      Having 2 90* turns in your vent pipe will add restrictions to the flow even though the length isn't that much. By how much, I couldn't really say but others might know the formula. But I'll wager you won't get near 1400 cfm with a 1400cfm blower.

      If it were me, I would put in the 10" duct and the larger blower. I can always use it on a lower speed. I'd rather have it and not need it than need it and not have it.

    • opaone
      6 years ago
      last modified: 6 years ago

      Measurement H = 9 7/8" appears to be the exhaust duct diameter.

      That blower measurement is 1000 CFM 'free air flow'. You will have static pressure losses in your hood and filter baffles, duct length, a bunch in those 2 90° elbows, and in the exhaust cap. You should ask Modernaire to tell you what the actual CFM will be for your specific installation. Eliminating those 2 90's or changing them to 45's (e.g., a diagonal run between them) could help.

    • PRO
      Victory Range Hoods
      6 years ago

      ILoveRed, my mistake. It looks like H (9 7/8) is in fact the duct size, I must have misread the chart. It is likely that the recommended duct size is 10" because the external blower that will be provided will be more than 1000 CFM. Do you know which external blower you will be using? Any external blower that is more than 1000 CFM will require a larger duct than 8". Any 90 degree elbows in the duct work will create a bit more resistance and reduce the suction by about 10%, and it would be better to use two 45 degree turns instead of a full 90 degree turn (as mentioned above). Let's say you are calculating the power of the external blower at 1000 CFM with an 8" duct, and two 90 degree elbows, you would likely achieve closer to 800 CFM because of the turns. If you use a 10" duct with a 1400 CFM blower, and the two 90 elbows, that would be close to 1100 CFM.

    • User
      6 years ago

      This is where I get pi$$y. Because any manufacturer that wants to say that they can put together a hood that will pull 190, 000 or whatever CFM, and then restricts the outflow duct to 8”, is a big fat prevaricator.

      That’s like trying to get a cookies and cream milkshake through a standard straw. The big pieces do not fit through the small straw.

      The realistic rating will always be determined by the most restrictive point. No matter what size fan you put on it, 8” can only allow so much air through.

      Is it enough air? In your case, yes, but I still am not happy about that bit of marketing deception.

    • chispa
      6 years ago

      ILR, when I ordered my Modern-Aire PS26 a few weeks ago, my appliance guy had me go with the 10" duct and a 1200 CFM internal blower. My hood vents straight out the back. My range is a 48" DF Wolf.

    • ILoveRed
      Original Author
      6 years ago

      Chispa...did you leave your Modernaire top opening at 12" depth or did you customize it to 13? I remember you customized the length. My length is customized to 27", but I chose not to customize the top opening because I didn't want to take a chance on changing the radius of the hood i.e.; the appearance. So, my 13" inset cabinets above the hood indent in 1" to accommodate the 12" hood depth. Cabinet order is in. Now I wish I would have just customized the top opening of the hood to 13" and left the cabinets at 13"

      If you left your hood opening at 12", then obviously the 10" duct size fits into the 12" opening.

      So frustrated. This is what I get for living in the middle of no where.

      thank you.

    • mishmosh
      6 years ago

      Being that your run is 12 ft and has two 90 degree elbows, i would only be happy with the 10" vent. If it went a short up and out, 8" would be plenty.

    • ILoveRed
      Original Author
      6 years ago

      Weedmeister..I apologize. On my screenshot I did cut off the headings. I should have just posted the link.

      8" duct requires an adapter...what kind of adapter? And where? Thank you for posting. Every post I'm reading on this thread is teaching me a little something that I didn't know. I think your advice is good about the larger vent and CFMs.

      thank you.

    • chispa
      6 years ago
      last modified: 6 years ago

      ILR, I did not customize the top depth, so it will sit slightly back from my top cabinets. My current/old hood has similar dimensions and it looks fine.

    • Anthony C
      6 years ago

      each 90 degree elbow adds about 12-15 equivalent feet of length. So if the run is 12 with two 90 degree, it becomes around a 36 ft run


    • ILoveRed
      Original Author
      6 years ago

      Opaone....I didn't realize that you lost CFMs with all of the things you mentioned? And that the CFMs are free air. I could be wrong and I want to give this retailer the benefit of the doubt but I don't think my kd even described my set up other than it had to go out the wall to vent. She seemed like a salesperson that had to go talk to someone with every question we asked. Not like the knowledgeable Modernaire people I have seen described in the past threads I have read about on GW. I certainly did any kind of "drawings" as described in these past threads.

      I don't know if we could change either of the 90 degrees to 45s. My brain is fried. Will you and others look at it?

      thank you.




    • ILoveRed
      Original Author
      6 years ago

      Victory...no problem. I did post a screwy screenshot with missing data, lol.

      I am going to post my plan and see if the gurus here have any ideas...thank you for helping.

    • ILoveRed
      Original Author
      6 years ago

      Sophie..

      "That’s like trying to get a cookies and cream milkshake through a standard straw. The big pieces do not fit through the small straw." I always love your analogies.

    • ILoveRed
      Original Author
      6 years ago

      Mishmash...thank you. I'm thinking the same thing more and more.

    • ILoveRed
      Original Author
      6 years ago

      Chispa...thank you. So the 10" duct fits in the 12" opening.

      What you did with your installation tells me I should have left my cabinets alone.

    • ILoveRed
      Original Author
      6 years ago

      Anthony...what??? Omgosh. What are the implications of this?

    • ILoveRed
      Original Author
      6 years ago

      Please forgive my chicken scratch on this screenshot of my kitchen plan. Any ideas to improve my venting?

      Information...Kitchen has 10 ft ceiling. Laundry room behind it will have 9' ceiling.

      The plan is to take the ductwork straight up above the rangetop, turn backwards 90 degrees into the room behind it which is the laundry room. (9' ceiling in laundry room or soffit to hide ductwork. I chose 9 ft ceiling).

      Another 90 degree turn with short run, straight to the outside of the house. The two 90 degree turns are very close to each other.

      if you look closely on the exterior walls, the windows are marked in yellow. So there is limited exterior wall space to mount this large exterior blower. Something like 21" by 27" in dimension.

      thanks for looking.


    • chispa
      6 years ago

      ILR, mine is actually venting straight out the back of the hood, but the order would not have changed if it had been vented through the top of the hood.

      I'm no expert and relied on my appliance guy (small, but high end shop) who checked/talked with my KD and his contact at Modern-Aire to make sure we had the right dimensions, etc. My vent run is barely a few inches, basically going straight through an outside wall.

      ILoveRed thanked chispa
    • kaseki
      6 years ago

      Unfortunately, I am late to this party. The preceding messages combine facts, truths in the practical sense but not facts in the universal sense, and some misunderstandings, perhaps. Let me offer some hopefully helpful clarifications, starting with these randomly selected observations.

      The major sources of air induced noise are blower blade tip turbulence, baffle turbulence, and then duct turbulence, usually in that order. Larger slower blowers can help mitigate blade tip turbulence; also an inline silencer can do this, as well as minimizing duct turbulence noise from the duct section beyond the silencer.

      Duct sizing should be selected: first to keep the effluent velocity in the 1000 to 2000 ft/min range to minimize grease build-up, then second to allow minimum restriction.

      1. One can move any reasonable amount of CFM through any reasonable duct (I'm going to skip defining reasonable) given a blower with the appropriate fan curve. At the limit, this blower would have to be positive displacement (like an air compressor) to move the air, and at some intermediate point, the blower would have to be a "screaming mimi" aerospace axial blower one wouldn't want to live in the same house with. Typical exterior residential blowers we know and love here are centrifugal type and have fan curves typified by those published by Broan, for example, available at their web site.

      The order of selection should be (1) choose the hood canopy to be large enough for good capture [H tan10 overlap]; (2) calculate the actual flow rate needed for good containment [90 CFM/sq. ft. unless there are mitigating circumstances]; (3) Select the largest practical duct size that will fit and that will meet the velocity bounds noted above; (4) Estimate all of the pressure losses (including in particular MUA) that will result at that flow rate [nearly impossible from available data -- why I suggest a 1.5 factor to find rated CFM]; (5) Find a blower that provides the desired actual flow rate at the anticipated pressure loss using the fan curves published for (at least some) manufacturers' blowers; (6) double check your MUA plan.

      Lets pretend here that the hood's capture aperture is 26 x 42 or 7.6 square feet. This calls for 680 CFM actual flow rate for good containment by the baffles of the plumes that are captured by the canopy aperture. Using the WAG 1.5X, this requires a blower rated at 1000 CFM. Now here is where some hand waving comes into play, as I have no idea what MUA concept is intended here, and for reasons I've already forgotten (have a virus) the hood may be restricted to an 8-inch duct with a longer than typical "effective" duct length. So maybe a 1200 CFM rated blower would be more appropriate to achieve 680 CFM. However, for that to be true, the household interior pressure would have to fall lower than desirable, so some care here is needed to meet MUA safety standards while trying to squeeze higher flow through a "constricted" duct.

      Note that for 680 CFM, 8-inch duct is going to have a flow velocity right up at the 2000 ft/min desirable limit, so 10-inch duct would normally be preferred.

      Back to hibernation; I may be able to study this in more depth much later today.

    • catinthehat
      6 years ago

      Good information as always kaseki, feel better.

      ILoveRed thanked catinthehat
    • ILoveRed
      Original Author
      6 years ago

      Thanks Kaseki..for looking at this when you are feeling so punk.

      i had to read the first 2/3 of your post several times as it is over my head. But the last third I think I got. Medical advice I could give you :-)

      it sounds like you are saying that I should insist on the 10" duct and the larger blower which in this case is this is (Abbaka) 1400 CFMs. ..if other things are addressed....

      specifically, my MUA. We have two geothermal units each with an HRV and a humidistat. The only gas we have in the house is the rangetop and a fairly large fireplace with gas insert.. Mild winters most of the time. Fireplace won't be on that much or for long.

      Hoping that the HRVs are considered adequate for MUA requirements. Honestly, I am an average cook and this rangetop vent will not be running at high speed for long periods of time. But, I still want it to be adequately sized from the get go.

      i hope I have addressed questions and interpreted you info correctly.

    • ILoveRed
      Original Author
      6 years ago

      Chispa...your installation sounds ideal. I didn't realize this hood could vent out the back rather than the top but of course it should.

      you have given me so much good advice, I'm going to offer you a tidbit. In my last house I had a 48" Wolf df (which was awesome btw, with no chipping issues) and it vented right out the back like yours will. It was very noisy. Granted my hood/vent system was poorly done. But I wonder if there is a noise reducing device that could be added to your system. Or is something built into your blower?

    • kaseki
      6 years ago
      last modified: 6 years ago

      Taking a break from not taking the break I should be taking....

      No air leaves via the hood and ducting that doesn't return to the kitchen some other way. So if 680 CFM is the intended flow rate, somehow 680 CFM at low pressure drop has to get back into the house and thence to the kitchen. Normally HRVs are designed to keep air fresh inside against nominal chemical outgassing; not deal with large blower flow rates.

      How low a pressure is too low? First, performance-wise, it has to not degrade the hood ventilation by more than allocated on the fan curve to the MUA function. Second, for safety (or at least for fireplace function) it should not violate any of the following that are relevant. (This image was supplied by someone being helpful here some weeks ago. Click to enlarge. Font selection by others.)

    • kaseki
      6 years ago

      So, the goal might be to find out the pressure loss of the HRV vs. flow rate. Half of 680 CFM would be expected to flow through each HRV. (Comfort analysis would include whether the HRVs/geothermal source can supply the 50 - 100 kBTU/hr that heating the air might require, depending on outside temperature. But that may be dependent on personal tolerance and cooking time requiring full blower power.) Can the HRVs supply enough air at low enough pressure drop to keep your fireplace from back-drafting?

    • mishmosh
      6 years ago

      lol. it all boils down to compromising with 8" or going for the safer choice of 10". mental masturbation is unnecessary.

      ILoveRed thanked mishmosh
    • opaone
      6 years ago

      Unless you have 2 very large HRV's and they offer bi-directional control then they will not provide MUA. Most HRV's are considerably less than 200 CFM and exhaust the same amount of air as they intake in which case they would provide zero MUA. HRV's with bi-directional control that allow them to turn off exhaust while still providing intake are somewhat rare and expensive.

      ILoveRed thanked opaone
    • chispa
      6 years ago

      ILR, happy to share and pay back all I've gotten from GW over the years. I hear you about the hood noise!

      I'm gutting the kitchen, but restricted to current footprint, an open floor plan and few walls, so many things will be similar replacements. The hood is one of them. I currently have a 48" Thermador hood that is noisy. New one will be moved just a few inches and vent out the same back wall. I am hoping that a fan motor that is 17 years newer will make less noise ... but no idea what has changed in fan motors, if anything.

      I'm also not someone who does lots of messy frying. Most of our meals are meats cooked on the grill outside eaten with steamed veggies and fresh salads.

    • PRO
      Trevor Lawson (Eurostoves Inc)
      6 years ago

      I feel the answer to the OP's question is...... Go with the 1400 cfm remote blower using a 10" round solid duct. Try to soften the turns a little if possible. In general 1400 cfm remote blower is as big as it gets for general home use. 10" round duct is a big as it gets for general home use. The combination of these two give you 2 of the 3 requirements for as good as it gets smoke and grease removal. The 3rd is capture area.

      Not sure why you would go with a 8" duct when you can use a 10" duct. No benefit only a possible lack of air flow on occupations when you really need it.

    • ILoveRed
      Original Author
      6 years ago

      Kaseki...thank you once again. Your explanation is once again over my head, lol. But will share it with my contractor. Now go rest!

    • ILoveRed
      Original Author
      6 years ago

      Chispa...I'm hoping with this new kitchen in this new house that I might become more adventurous in my cooking. I'm afraid I passed my lack of culinary ability to my oldest dd as she is not so great. Hr dh otoh is fabulous.

      Looking forward to seeing the results of your big remodel. Hope you will share.

    • ILoveRed
      Original Author
      6 years ago

      Trevor...thank you for the simple and direct advice.

      "Not sure why you would go with a 8" duct when you can use a 10" duct. No benefit only a possible lack of air flow on occupations when you really need it."

      my kd was told (by the Modernaire Retail salesperson that she is dealing with) that the 10" duct will not fit in the ps26 12" top. This is what really started this whole thing.

      if I am making a big purchase I want to make sure I'm getting good advice and not messing things up. Been there done that.


    • opaone
      6 years ago
      last modified: 6 years ago

      One benefit to a smaller duct and blower is less grease build-up in the duct when running on slower (quieter) speeds. As @kaseki mentioned earlier, you ideally want to maintain certain velocities in the duct to keep grease from dropping out of the air stream and collecting in the duct. (This is btw, the one advantage to the otherwise not so great VAH in that it does seem to do a fairly good job of removing grease from the effluent prior to the duct.)

      This is not to say that you shouldn't do 10" + 1400 but to note that bigger is not always necessarily better.

      ILoveRed thanked opaone
    • chispa
      6 years ago

      ILR, you can come out to LA for a few days and I'll drive you to the Modern-Aire office/factory! We'll camp out in their tiny foyer till they answer all your questions!

      We can then go backsplash shopping!

      If you aren't sold yet ... it is also sunny and 70F!!!!

    • ILoveRed
      Original Author
      6 years ago

      Opaone...

      "One benefit to a smaller duct and blower is less grease build-up in the duct when running on slower (quieter) speeds. "

      Well that gives you food for thought. So how much grease and other matter actually gets through the hood and filter system and into the duct and duct run? I've never really considered "stuff" passing through into a ducting system that is enclosed in drywall and essentially becomes inaccessible for the life of the home.

      wow.

    • ILoveRed
      Original Author
      6 years ago

      Chispa...oh my. That sounds fun since our Illinois weather right now just plain sucks. Looking forward to the day we backsplash shop because that means we are both almost finished.

      I am sure I will get all of these questions taken care of and get this thing ordered this week and next week I will have another issue to obsess over ;-)

      Nothing as pretty as these hoods.......


    • kaseki
      6 years ago
      last modified: 6 years ago

      ILoveRed asked:

      "Well that gives you food for thought. So how much grease and other
      matter actually gets through the hood and filter system and into the
      duct and duct run? I've never really considered "stuff" passing through
      into a ducting system that is enclosed in drywall and essentially
      becomes inaccessible for the life of the home."

      Based on my experience with older ducting used for many years, the actual amount of grease generated in residential use is so much less than in commercial settings (where ducts have to be inspected and cleaned) that you should predict that a film will build up over time and rancidify (turn solid). The fire hazard should be minimal, but not negligible, so always cook with the baffles present.

      If I were forced to route duct where it is generally inaccessible, I would try to at least provide access at accessible points for inspection if it were needed. Handling 10-inch duct is a wrestling match in any case, so one hopes to never need to look. The blower end and the hood end can be inspected as needed.

      As may be seen in the example below, a lot gets past baffles, but particle sizes are mostly small and usually will not collect in the ducts unless they are quite cold or the velocity is so high that wall impingement is forced.

      The latest that I know about relating minimum air velocity to temperature is provided by this article: http://www.culinairesystems.com/files/ASHRAE_Article_Exhaust_Velocity.pdf

      I am reminded by it that going as low as 500 ft/min is now allowed by NFPA 96

    • Chris Balster
      4 years ago

      I will start a new thread if that's the protocol but I have a similar question. I moved into a 115 year old home with a first floor kitchen vented to the roof. The building is 3 floors+attic, and the hood has a 7x7" duct opening to the roof. The run of the duct is roughly 40' vertical without any turns (that I know of). On the roof there is a several decades old mushroom motor that works but does not create much ventilation when we cook. We tend to cook with a wok so we want better ventilation. If we were to replace the motor on the roof, what type of CFM could our ducting support? Thanks for reading and looking forward to comments.

    • kaseki
      4 years ago
      last modified: 4 years ago

      Without addressing the actual flow profile, a 7 x 7 duct is 49 square inches, or .34 square feet in sectional area. At 1000 ft/min, the flow could be 340 CFM. At 2000 ft/min 680 CFM. I would consider that the upper upper limit. Also, the pressure loss will be higher for a square duct than a round duct of the same cross section, so blower selection should be based on total pressure loss calculated from the duct shape and length (or more conservatively, a 7-inch diameter round duct), the hood baffle pressure loss vs flow rate (may have to be estimated based on baffle manufacturer data), and pressure loss in the make-up air system (which needs to be defined during the process of dealing with the hood system).

      Even so, the achieved flow rate might be marginal for wok cooking. You want around 90 CFM/sq. ft of hood entry area, and a hood that at a minimum overlaps the wok by enough to capture the rising and expanding plume, larger than the wok by about 1/6 the distance between wok and hood entry aperture around the hood boundary. Of course, this should be done for every burner to define the hood extent.

      Do you have a practical way to go out a wall, or to expand the existing duct chase sectional area to accommodate a 8 to 10 inch duct?

    • Chris Balster
      4 years ago

      With a unit above us and an HOA it's hard to start messing with the ceiling to shorten the duct run and exit through the side of the house. Our existing range hood, a real copper craftsman piece, has the 7x7 exit to the vertical duct, so I guess our options may be limited.

    • kaseki
      4 years ago
      last modified: 4 years ago

      Any continuation of Chris' specific questions by me will be in the thread linked by:

      https://www.houzz.com/discussions/5848031/external-blower-cfm-question-for-old-house-kitchen-40-vert-roof-duct#n=4

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